When an automotive fuel tank is filled, air laden with fuel vapor is displaced by fuel. The fuel tank is connected by a vent to the atmosphere to provide pressure equalization both during fueling, and as fuel is used by the engine. A carbon canister is placed between the fuel tank and the atmosphere to remove and temporarily store fuel vapors from the air before being exhausted. The fuel vapors are subsequently delivered to the engine for combustion during a periodic carbon canister purge operation. The emission system components, including the fuel vapor storage system, are periodically tested onboard the vehicle to ensure proper operation and to detect any leaks that may allow fuel vapors to enter the atmosphere. The vapor purge system may be tested for leaks by creating a vacuum or reducing pressure relative to atmosphere and observing the change in pressure over time. If the system pressure or vacuum remains constant or slowly changes, no leak is indicated. However, a rapid change in system pressure toward atmospheric pressure indicates a leak.
In a conventional vehicle, the vacuum in the intake manifold during an engine idle condition may be applied to the vapor purge system to determine if leaks are present using a system integrity test. However, for an engine system disposed in an HEV, engine idle is generally avoided because it is an inefficient operating condition. Of course, the engine in an HEV may be commanded to operate at idle to facilitate a system integrity test of the purge system; however, this consumes additional fuel. Alternatively, the system integrity test may be conducted during a steady-state cruise condition of the HEV with the engine running; however, the system integrity test may take as long as three minutes. During this time the test may have to be aborted due to a changing engine condition or a changing vehicle path causing fuel in the tank to slosh and release a large amount of vapors, thereby causing a pressure change and falsely indicating a leak. In another alternative, a vacuum pump is provided; however, this requires an additional piece of hardware in the HEV and is costly.